Prussian Blue Mg-Li Hybrid Batteries

Abstract

Mg batteries have received attention as a potential candidate for energy storage. The main attraction relies on the use of a Mg negative electrode, which is inexpensive, safe to handle and store, and offers high volumetric energy density together with dendrite free deposition during electrochemistry processes.(1) However, sluggish Mg2+ diffusion in most solid structures has hindered development of the positive electrode material, and thus the entire system. Nevertheless, this issue can be addressed by coupling the Mg negative electrode with a Li insertion positive electrode through a dual-salt electrolyte, so that the advantages of the Mg anode is preserved and facile monovalent cation diffusion in the positive electrode is accomplished.(2) In this presentation, we will discuss two Mg-Li hybrid batteries that employ Prussian Blue analogues (PBA) with different water contents – Fe[Fe(CN)6]0.95·2.3H2O (23-PBA) and Fe[Fe(CN)6]0.95·0.7H2O (07-PBA) – as the positive electrode materials in the all phenyl complex (APC) (3) + LiCl dual salt electrolyte (Figure 1a). The PBA was specifically chosen due to its open tunnels in the crystal structure that provide various ion pathways, as well as robust Fe-CN bonds which ensure a high resistance to chlorine corrosion due to the electrolyte. The materials were tested at a current density of 10 mA g-1 (~ C/10, 1C = 1e-/PBA f.u.), resulting in ~ 130 mAh g-1 specific capacities for both PBAs. Two well defined voltage plateaus were observed at 2.6/2.0 V (vs. Mg) for 23-PBA and 2.3/2.0 V for 07-PBA (Figure 1b), associated with the two distinct Fe cations bound to C and N, respectively. The higher voltage observed for 23-PBA resulted from the additional stability of the lithiated phase resulting from coordinating the inserted Li+ with structural water. The ± 0.1 V voltage for metal stripping/plating on the Mg negative electrode (Figure 1b inset) suggests that the reduction of Li+, which would typically commence at -0.7 V, did not occur. Coulombic efficiency of 99% up to 300 cycles for 07-PBA was observed, in contrast to the much poorer performance of 23-PBA (Figure 1c). The lower capacity retention of 23-PBA originated from its structural water which remained in the material during the first cycle but dissolved into the electrolyte upon prolong cycling, as proven by ex-situ FT-IR. The detailed Li+ insertion mechanism was studied by in-situ XRD, where similar results were obtained for both materials. A decrease of cell parameter was observed on the higher voltage plateau, resulting from the additional electrons introduced to the Fe-C bonding orbital. At the beginning of the lower voltage plateau, phase separation occurred but the following charge illustrated the reversibility of the entire process that represents excellent reversibility of Li+de/intercalation into the structure. Even after prolonged cycling in the hybrid cell, a dendrite free surface was obtained on the Mg negative electrode, indicating a very different electrodeposition process is prevalent compared to that of Li metal. As such, the primary advantages of the Mg negative electrode are preserved. Thus, the Mg-Li hybrid system with a PBA positive electrode provides a new direction to explore “high voltage” Mg batteries. Other promising positive electrode Mg2+insertion materials will be also discussed in our presentation that do not rely on the hybrid concept.